1. Field of the Invention
The present invention relates to manufacturing of glass.
2. Description of Related Art
When glass is continuously mass-produced, the following process is generally adopted. Glass material is melted in a melting tank having wall surfaces made of refractory material comprising refractory ceramic, and then the molten glass is fined. The fined molten glass is formed in a desired shape. Here, in order to melt the glass material or to maintain the molten state of the glass, flame heating in which a fossil fuel is burned with a heavy oil burner, a gas burner or the like, or ohmic heating in which electrodes are used for making electric current flow through the molten glass to heat it. When ohmic heating is used, rod-like or block-like electrodes are used. It is to be noted that such electrodes are, for example, placed on a bottom wall or a side wall of a melting tank (see, for example, JP 2003-183031 A, JP 2005-225738 A and JP H05-4820 A), or supported by electrode supporting frames which protrude vertically from a bottom wall of a melting tank (see, for example, JP 2005-53757 A).
As necessary, in a melting tank, various members are placed at positions in contact with the molten glass, including a temperature sensor for monitoring the temperature of the molten glass (see, for example, JP 2003-286031 A and JP 2005-225738 A), a bubbling member for ejecting gas in the molten glass for the purpose of homogenizing the molten glass (see, for example, JP 2005-53757 A and JP 2006-516046 A), and a pipe for introducing molten glass into the melting tank from another melting tank or for discharging the molten glass in the melting tank to the outside (see, for example, JP 2006-516046 A). It is to be noted that, because these members are required to be heat-resistant and corrosion-resistant, these members are typically formed of or coated with a metallic member made of Pt, Pt alloy, or the like which is resistant to heat and corrosion by molten glass.
Exemplary glass which is mass-produced by a glass manufacturing apparatus using the melting tank as described above includes not only glass for a vehicle and glass as a construction material, but also glass for an information recording medium used in a magnetic recording medium or the like, glass for a display such as a liquid crystal display, and glass used in precision equipment such as electronic equipment or optical equipment, for example, glass for an optical component such as a lens.
In glass manufactured by a glass manufacturing apparatus using the melting tank as described above, various foreign matters may be mixed in the process of the manufacture. Such foreign matters may cause a product defect. For example, in a case of a glass substrate for a magnetic recording medium, metal oxide particles which originate from the refractory forming the melting tank and which are taken in the glass may present themselves as minute protrusions on a surface of the substrate. It is known that such protrusions cause protrusions to be formed accordingly on a surface of the recording medium, which causes a head crush in which a head crushes against a protrusion on a surface of a magnetic disk (see JP 2003-137557 A).
A product defect caused by foreign matters is not limited to glass for a magnetic recording medium. In particular, in glass used in precision equipment described above, irrespective of the material of the foreign matters, minute foreign matters which are submicron-to-several-micron-sized and which exist in the glass or in the vicinity of a surface of the glass may cause a product defect.
For example, in a magnetic recording medium, if a minute protrusion originating from foreign matters with height about several nanometers exists on a surface of a glass substrate for the magnetic recording medium, a problem called thermal asperities may be caused in addition to the head crush described above. Further, if a vestige of a foreign matter dropped off exists on a surface of an information recording medium such as a magnetic recording medium, such a recording medium may become inferior. In a liquid crystal display, if a protrusion originating from foreign matters with height a little over a hundred and ten nanometers exists on a surface of a glass substrate for the display, problems such as a break in electrode wiring of a pixel arise. Such problems may also arise on other displays than liquid crystal displays, affected by foreign matters having somewhat different size. In an optical device, foreign matters lager than the order of the wavelength of the light used cause problems such as light scattering.
As described in JP 2003-137557 A, the inventors of the present invention seeks to eliminate foreign matters in glass used for various applications, using a technology for suppressing defects due to foreign matters which originate from the refractory forming the melting tank, and the like. However, even when foreign matters originating from the refractory are suppressed, in mass production of the glass for a long period of time, the number of product defects caused by particles of metal such as Pt may suddenly increase with time at a certain point and become non-negligible in securing the yield of the product.